1. Technical Field
The present invention relates to a method for fabricating an image sensor, and more particularly, to a method for fabricating an image sensor whose quality is improved by improving a formation mechanism of a gate insulating layer pattern and improving a doping sequence of impurity ions of a gate electrode pattern.
2. Discussion of the Related Art
Electronic appliances such as video cameras, digital cameras, personal computers with a small sized camera, and cellular phones with a small sized camera have been widely used. These electronic appliances use image sensors to capture images. Conventional image sensors include a charge coupled device (CCD) and a CMOS image sensor. A CCD has several drawbacks in that a high driving voltage and an additional circuit are required and the process cost is high. In contrast, a CMOS image sensor based on CMOS circuit technologies is generally used to replace a CCD, because a CMOS image sensor requires low driving voltage and no additional circuit is required, and is cheaper to manufacture.
A method for fabricating a conventional image sensor is described with reference to FIG. 1A to FIG. 1E.
As shown in FIG. 1A, an active region of a semiconductor substrate 1 is defined by a device isolation region 2, and a screen insulating layer 4 is grown on the surface of semiconductor substrate 1. A region for photodiode in the active region is shielded by a photoresist pattern 101, and P type impurity ions are doped into the region exposed by photoresist pattern 101 to form a well 3. As shown in FIG. 1B, N type impurity ions are counter-doped into an upper portion 3a of well 3 for a transistor. As shown in FIG. 1C and FIG. 1D, after photoresist pattern 101 and screen insulating layer 4 are removed, an insulating layer 5a and a polysilicon layer 6a are formed on the entire surface of semiconductor substrate 1. Insulating layer 5a and polysilicon layer 6a are then patterned to form a gate insulating layer pattern 5 and a gate electrode pattern 6 over portion 3a of well 3 region for transistor. As shown in FIG. 1E, spacers 7 are formed on the sides of gate electrode pattern 6. Then, N type impurity ions are doped into the gate electrode pattern 6 and semiconductor substrate 1 using spacers 7 as a mask, thereby forming source and drain diffusion regions 8 and 9 on the sides of spacers 7. As a result, a transistor 10 including gate insulating layer pattern 5, gate electrode pattern 6, and source and drain diffusion regions 8 and 9 is formed. Finally, N type impurity ions and P type impurity ions are doped into the region for photodiode to form an N type diffusion layer 11 and a P type diffusion layer 12, which together constitute a photodiode 13.
Transistor 10 may serve to enlarge a depletion region of photodiode 13 by the on/off operations thereof and it is important for transistor 10 to have a small threshold voltage to achieve a good quality of the image sensor. The doping of N type impurity ions into upper portion 3a of well 3 as shown in FIG. 1B is generally performed to reduce the threshold voltage of transistor 10. However, the doping in upper portion 3a of well 3 results in leakage currents between well 3 and source and drain diffusion regions 8 and 9, which in turn deteriorates the quality of the image sensor.
Also, as shown in FIG. 1E, impurity ions are doped both into semiconductor substrate 1 to form source and drain diffusion regions 8 and 9 and into gate electrode pattern 6. To avoid damage that may occur in the photodiode region, source diffusion region 8, which is adjacent to photodiode 13, is doped at a concentration lower than drain diffusion region 9.
Consequently, the impurity ion concentration in a region 6a of gate electrode pattern 6 adjacent to photodiode 13 is different from that in a region 6b of gate electrode pattern 6 adjacent to drain diffusion region 9, which also deteriorates the performance of gate electrode pattern 6 as well as the quality of the image sensor.